The present invention relates to a method and device for testing conductive articles such as bodies, metal sheets, blanks, wires, tubes liquids, powders etc. with respect to changes such as errors, defects, deviations, dimensional changes, changes in the electrical properties, including changes caused by external influence, such as speed, force etc. The method and device according to the invention provides for detection and evaluation of the change, for example its type, size, position etc. A detectec change, for example originating from dimensional changes, is adapted to be measured thereby providing a measure of the dimension and the dimensional change respectively. If the change originates from mechanical stresses in the test specimen, for example from external force applied to the test specimen, the stress is measured and a measure of the force and change of force is obtained. If said change originates partly or completely from relative motion between the test specimen and the pickup probe a measure of the speed of the test specimen passing said probe is obtained. Integrating said speed will provide a measure of the length of the specimen which has passed the probe. The change or error may also originate from a change from presence of material to non-presence of material in which case the device is suitable as metal detector.
Devices for performing nondestructive testing are previously known and include at least one pickup probe excited by alternating current. The magnetic field of the coil generates eddy currents in the test specimen and changes in the test specimen appear in the form of changes in the distribution of the eddy currents. The eddy currents are detected by pickup probes which in the known devices often form a part of a balanced bridge circuit or other similar unbalance circuits.
Said known devices are often of the self comparison type, that is, a section the specimen constitutes a reference or normal for another section thereof which implies that such devices are not suitable for the detection of so called oblong defects such as long cracks etc.
It is also previously known to make use of devices wherein the pickup probe is excited with multiple frequencies, compare for instance the method of H. L. Libby described in Introduction to Electromagnetic Nondestructive Test Methods/Whiley o Sons, Inc. 1971. Said method is based upon Fourier transformation.
In these known devices it is difficult if not impossible to separate, in a simple and practical way, the so called error signal, that is the signal originating or emanating from a change, from position signals originating from the position of the pickup probe in relation to the test specimen. This implies that these devices do not operate satisfactory when the test specimen for example is vibrating. However, in most practical applications the test specimen is vibrating which, when so called through pickup probes are used, implies that the movement of the test specimen in relation to the radial extent of the pickup probe will give rise to a position signal modulated by said movement, said modulated signal often dominating over the error signal emanating or originating from said defect. When using so called surface pickup probes a modulation caused by the position variation between pickup probe and test surface is obtained in a corresponding manner.
In the Swedish printed and published Swedish Pat. No. 75.07857-6 there is disclosed a device which provides for detection of changes of an article independent of the position of the pickup probe in relation to the test specimen. The device described therein comprises like the present invention, at least one pickup probe or at least one wire loop which is excited with at least two different frequencies. Like the present invention the device is calibrated against for example a reference specimen. This calibration takes place at a certain distance between the reference specimen and the pickup probe. The calibrated device is then used for testing. When the pickup probe is at a certain distance (point P.sub.min in FIG. 4) which is depending on the change, from the test specimen the change cannot be detected. This is of course a drawback. The present invention is directed to solve this problem and other problems related thereto.